Rabble arm for a furnace

ABSTRACT

A rabble arm for a furnace includes an elongated metallic support core, at least one rabble tooth having a rabble portion and a fixing portion and a fixing device co-operating with the fixing portion for fixing the rabble tooth to the elongated metallic support core. The fixing portion includes a through hole through which the elongated metallic support core axially passes. The fixing device co-operates with the fixing portion around the through hole for fixing the rabble tooth to the elongated metallic support core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of International ApplicationNo. PCT/EP02/05114 filed on Apr. 19, 2002, and Luxembourg PatentApplication No. 90 762 filed on Apr. 20, 2001.

FIELD OF THE INVENTION

The present invention relates to a rabble arm for a furnace, inparticular a multiple hearth furnace.

BACKGROUND OF THE INVENTION

A multiple hearth furnace comprises an upright cylindrical furnacehousing that is divided by a plurality of vertically spaced hearthfloors in vertically aligned hearth chambers. A vertical shaft extendscentrally through the hearth chambers, passing through each hearthfloor. In each hearth chamber at least one rabble arm is fixed to thevertical shaft and extends radially outside there-from over the hearthfloor. Such a rabble arm is provided with rabble teeth, which extenddown into material being processed on the hearth floor. As the verticalshaft rotates, the rabble arm moves over the material on the respectivehearth floor, wherein the rabble teeth plough through the material andmix the latter. Depending on the angle of inclination of the rabbleteeth, the material will be moved radially inwardly toward the verticalshaft or outwardly therefrom. Drop holes are provided in each hearthfloor, alternately in the inner zone of the hearth floor (i.e. near thevertical shaft) or in the outer zone of the hearth floor (i.e. near thecylindrical furnace housing). Material falling on the inner zone of ahearth floor is moved by the rabble arm radially outwardly over thishearth floor, until it drops through a drop hole in the outer zone ofthis hearth floor on the outer zone of a hearth floor located directlybelow. On this lower hearth floor, material is moved by the rabble armradially inwardly until it drops through a drop hole in the inner zoneof this hearth floor on the inner zone of the next lower hearth floor.Thus, material to be processed is caused to move slowly along aserpentine path through the vertically aligned hearth chambers of thefurnace.

It is a fact that multiple hearth furnaces possess major advantages overother solid material processing furnaces, such as rotary hearthfurnaces, rotary kiln furnaces and shaft furnaces. By allowing a controlof different hearth atmospheres and temperatures in the verticallyaligned hearth chambers, they allow a very close control of the processinside the furnace. Other advantages of multiple hearth furnaces lie intheir ability to maintain the processed materials in mixed conditionthroughout their passage through the furnace and to warrant a veryintense exposure of the solid materials to process gases in a controlledgas/solid material counter flow within the furnace. Nevertheless, sincetheir invention at the end of the nineteenth century, multiple hearthfurnaces have only found very few applications in solid materialprocessing. A reason for this lack of confidence in multiple hearthfurnaces is that it has never been possible to warrant a problem-freeoperation of a multiple hearth furnace over longer periods.

The most exposed elements in a multiple hearth furnace are the rabblearms with their rabble teeth. These rabble arms and rabble teeth aresubjected to severe temperatures and severe mechanical constraints in afurnace atmosphere that is usually very corrosive. Already in very earlymultiple hearth furnaces, the rabble arms included a water or gas cooledcast iron support structure, and the rabble teeth were conceived asexchangeable wear parts. Such an exchangeable rabble tooth generallyincludes a dovetail interlocking element at its upper portion engaging,in a form-fit relationship, a corresponding groove at the underside ofthe cooled metallic support structure.

An allegedly improved design of a rabble arm was disclosed in 1968 inU.S. Pat. No. 3,419,254. This rabble arm includes a hollow cast ironcore obtained by mould casting. It is divided by a central web into twoseparate passageways for cooling air. The teeth of the arm are formed ofa ceramic material. They have an upper fixing portion with a pair ofinwardly facing hook-like interlocking elements, which are dimensionedto fit loosely over lower horizontal flanges laterally protruding fromthe underside of the metallic core. In order to provide an insulatingand shock absorbing tight connection between the rabble teeth and themetallic core, a fibrous insulating material is interposed between thehook-like formations and the lower horizontal flanges. To complete theinsulation of the metallic core, an inner layer of fibrous insulation isplaced over the top part of the metallic core, and an outer solidinsulation is finally placed on top of the inner fibrous insulation.Lugs on the metallic core prevent the cover from moving longitudinallywith respect to the metallic core. In an alternative embodiment, aplurality of wire-like prongs is welded to the metallic core along itssides and top. Thereafter, a layer of fibrous insulating material ispressed down over the prongs so that it lies snugly over the top of thecore. A castable insulation is finally cast over the exterior of therabble arm, where it is held in place by the wire-like prongs.

A first drawback of known rabble arms is a rather high frequency ofteeth breaks in the region of their dovetail or hook-like fixingportion. It will be noted in this context that a break-off of a singletooth may cause severe damages to the rabble arms of the hearth chamber,because the broken off rabble portion is an obstacle for the remainingrabble teeth and may cause a break-off of further teeth or even acollapse of whole rabble arms.

A further drawback of known rabble arms is their insufficient protectionagainst high temperatures. The thermal insulation of known rabble armsis indeed deficient in respect of many aspects. It will be noted e.g.that the underside of the rabble arm, which is exposed to the highestheat load, has the poorest insulation. Furthermore, it happens quiteoften that the thermal insulation of a rabble arm falls off alreadyafter a short operation period of the furnace. As an overhauling of thethermal insulation of a rabble arm requires the removal of the rabblearm, the operator of the furnace usually runs usually the risk not torepair the thermal insulation of the rabble arms until the next majoroverhauling of the furnace, which requires anyway the dismounting of therabble arms. In the meantime, the unprotected metallic core of therabble arm is however exposed to a much higher thermal load than thethermal load it is designed to withstand.

Still another drawback of present rabble arms is a poor wear resistanceof their rabble teeth. Indeed, most rabble arms are still equipped withcast iron rabble teeth, which become subject to rapid wear undercorrosive hearth atmospheres and/or high hearth temperatures. Ceramicrabble teeth would of course be more wear resistant in such atmospheres,but the manufacture of ceramic form pieces of the size of a rabble toothis still a rather expensive operation. It follows that the use ceramicrabble teeth is normally economically not justified. Furthermore,ceramic rabble teeth may be very wear resistant but they havenevertheless a low ductility, i.e. they are often subjected to breakagein particular in the region of their dovetail or hook-like fixingportion.

Further rabble tooth structures are disclosed in following documents:

U.S. Pat. No. 1,468,216 discloses a cooled rabble tooth structurecomprising a cylindrical hub as fixing portion and a hollow tooth bladeas rabble portion. The hollow tooth is integrally cast with thecylindrical hub. The cylindrical hubs are assembled end to end on theelongated metallic support core of the rabble arm and cooperatetherewith to direct a cooling medium into the hollow teeth.

DE 389355 discloses a rabble tooth structure comprising a sleeve with atrapezoidal cross-section as fixing portion and at least one rabbleblade that is integral with the sleeve and projects from a side wall ofthe latter. The rabble tooth structure is made of a acid proofrefractory material.

U.S. Pat. No. 1,687,935 discloses a rabble tooth structure comprising adovetail fixing portion engaging a corresponding groove at the undersideof the metallic support core of the rabble arm.

OBJECTS AND SUMMARY OF THE INVENTION

A first technical problem underlying the present invention is to improvein a cost effective way wear and break-off resistance of the rabbleteeth in a rabble arm.

Such a rabble arm for a furnace comprises an elongated metallic supportcore, at least one rabble tooth having a rabble portion and a fixingportion and fixing means co-operating with the fixing portion for fixingthe at least one rabble tooth to the elongated metallic support core. Inaccordance with an important aspect of the present invention the atleast one rabble tooth has the form of a plate, wherein the fixingportion does not include a dove tail, hook or sleeve like fixingelement, but its fixing portion is simply formed by a portion of thetooth plate including a through hole through which the elongatedmetallic support core axially passes. A fixing means co-operates withthe fixing portion around the through hole for fixing the rabble toothto the elongated metallic support core. It follows that the fixingportion of the rabble tooth does no longer include recesses thatgenerate stress concentrations that are probably responsible for mostbreak-offs of rabble teeth. Furthermore, the fixing means can co-operatewith the whole fixing portion around the through hole for fixing arabble tooth to the elongated metallic support core. This means that—incomparison with a rabble tooth that is fixed by means of a dove tail,hook or sleeve like fixing element—a better distribution of stresses canbe achieved in the fixing portion.

Another advantageous aspect of the new fixing portion is that the shapeof the rabble tooth can be very simple. It may for example have the formof a flat plate with an oval through hole. A direct consequence of thesimple shape of the rabble tooth is that it can for example be made of aceramic material at reasonable costs. In conclusion, the presentinvention allows to have at reasonable costs rabble teeth having a goodwear resistance and being far less subjected to break-off than prior artrabble teeth.

The rabble tooth may have a constant thickness over its height. However,the thickness of the rabble tooth may also be varied over its height, soas to achieve a substantially uniform stress distribution in the rabbletooth. It will be appreciated that such rabble tooth of uniform strengthhas a reduced weight with regard to a rabble tooth with a constantthickness designed to resist to the same forces.

Instead of making the whole rabble tooth of a single material, it isalso possible to conceive a rabble tooth having a fixing portion thatconsists of a first material and a rabble portion that consists of asecond material, wherein the first material is preferably more ductilethan the second material. In such a composite rabble tooth, the fixingportion has the advantage to have a good resistance against breakage (itdeforms plastically instead of breaking), whereas the rabble portion ismore wear resistant. It will be noted that the first material can forexample be a cast steel and the second material a ceramic material. Therabble tooth may also include a core made of cast steel, which extendsover the fixing portion and the rabble portion and is provided with aceramic jacket in the rabble portion.

In a preferred embodiment of the rabble arm, the fixing means comprisesa teeth support sleeve slipped over the elongated metallic support coreand engaging the through hole in the fixing portion of the rabble tooth.Such a teeth support sleeve preferably supports several rabble teeth byengaging their through holes. It provides advantageously a form-fit withthe elongated metallic support. In particular, the outer cross-sectionof the teeth support sleeve and the through hole have for example bothan oval shape, so that the teeth support sleeve is blocked in rotationon the elongated metallic support core. The teeth support sleeveprovides advantageously a form-fit with the through hole of the rabbletooth.

The teeth support sleeve may be thermally insulated, which allows tohave a continuous insulation of the elongated metallic support core thatis not interrupted by the fixing elements of the rabble teeth. It willbe appreciated that such a continuously insulated teeth support sleeveis thermally more efficient and moreover less exposed to a fall-off thanany prior art insulation of a rabble arm. Furthermore, an overhauling ofthe thermal insulation of a rabble arm requires no longer the removal ofthe rabble arm, the insulated teeth support sleeve can be simply slippedover the elongated metallic support core, thereby replacing the rabbleteeth and the thermal insulation in one operation from the outside ofthe furnace.

A preferred embodiment of such a teeth support sleeve includes an innermetallic sleeve and an outer metallic sleeve, an insulating materialbetween the inner metallic sleeve and the outer metallic sleeve and ashock absorbing cushioning layer on the outer metallic sleeve. Thecushioning layer is engaged by the edge of the through hole in thefixing portion of the rabble tooth, whereby this edge is efficientlyprotected against mechanical damages, and a shock on one rabble tooth isabsorbed by the cushioning layer and not transmitted to the rest of theteeth support sleeve and the metallic support core. Furthermore, thecushioning layer helps to further improve the thermal insulation of therabble arm. It is recommended to make both the outer tube and the innertube of stainless steel. Such stainless steel tubes form an efficientcontinuous sheeting of the rabble arm against an excessive exposure tocorrosive gases.

A preferred embodiment of the teeth support sleeve further includesarmature elements protruding from the outer metallic sleeve through theshock absorbing cushioning layer and a layer of castable refractory onthe shock absorbing cushioning layer, wherein the fixing portion of therabble tooth is embedded in the refractory layer. It will be appreciatedthat such a teeth support sleeve can easily be conceived as aprefabricated unit to be simply slipped on the elongated metallicsupport.

A preferred embodiment of the metallic support core comprises twosuperimposed outer tubes, which are rigidly fixed together. Thesesuperimposed outer tubes are advantageously formed of centrifugally caststeel pipes. It will be appreciated that these outer tubes of themetallic support core have a very homogeneous structure that issubstantially free from casting cavities and other casting defects,which are unavoidable in a prior art support core obtained by mouldcasting. In conclusion, the metallic support core is-despite possiblylower manufacturing costs-less exposed to mechanical failures andcorrosion than any other metallic support core of prior art rabble arms.

In order to optimize cooling of the rabble arm, each of the outer tubesadvantageously includes a coaxial inner tube, which is arranged in theouter tube so as to delimit therein an annular gap for a coolant flow.Thus, it is warranted to obtain an efficient and homogeneous cooling ofthe outer tubes. The cooling effect may further be improved atreasonable costs, by simply arranging a wire is the aforementionedannular cooling gap, so as to define a spiral flow path for the coolantin the annular cooling gap.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1A: is a longitudinal section through the rear end of a rabble armin accordance with the present invention;

FIG. 1B: is a longitudinal section through the front end of the rabblearm of FIG. 1A;

FIG. 2: is a cross-section along section line 2—2 in FIG. 1A;

FIG. 3: is a longitudinal section through a teeth support sleeve of therabble arm of FIG. 1;

FIG. 4: is a top view of the teeth support sleeve of FIG. 3;

FIG. 5: is a cross-section along section line 5—5 in FIG. 1A;

FIG. 6: is a front view of a rabble tooth; and

FIG. 7: is a vertical section through the rabble tooth of FIG. 6.

DETAILED DESCRIPTION OF A THE INVENTION

FIGS. 1A and 1B show both end portions of an elongated rabble arm inaccordance with the present invention. This rabble arm is to besupported by a vertical shaft in a multiple hearth furnace. It includesan elongated metallic support core 10, i.e. a kind of hollow cantileverbeam that is fixed at one end with the help of a fixing flange 12 to thevertical shaft, so as to extend radially outside therefrom over a hearthfloor to the furnace wall. The object of this metallic support core 10is to support radially spaced rabble teeth 14 ₁, 14 ₂, 14 ₃, 14 ₄, whichextend down into the material on the hearth floor. As the vertical shaftin the hearth furnace rotates, the rabble arm moves over the material onthe respective hearth floor, wherein the rabble teeth 14 ₁, 14 ₂, 14 ₃,14 ₄ plough through the material on the hearth floor. Depending upon theangle of inclination of the rabble teeth 14 _(i) with respect to thelongitudinal axis of the rabble arm (see FIG. 4), the material will bemoved radially inwardly toward the vertical shaft or radially outwardlytherefrom.

The metallic support core 10 comprises two superimposed outer tubes 16,18, which are welded together (see in particular FIG. 2) and welded atone end to the fixing flange 12 (see in particular FIG. 1A). Each ofthese tubes 16, 18 is preferably made up of one or more centrifugallycast steel pipes. It will be appreciated that the centrifugally caststeel pipes have a very homogeneous structure that is substantially freefrom casting cavities and other casting defects, which are unavoidablein a prior art support core obtained by mould casting. In conclusion,the metallic support core 10 is—despite possibly lower manufacturingcosts—less exposed to mechanical failures and corrosion than any othermetallic support core of prior art rabble arms.

Each of the outer tubes 16, 18 includes a coaxial inner tube 20, 22,which is arranged in its outer tube 16, 18 so as to delimit therein anannular gap 24, 26 for a coolant flow. A wire 28, 30 is arranged in eachof the annular gaps 24, 26 so as to define a spiral flow path for thecoolant. Through an inlet opening 32 in the flange 12 and an inletchamber 33 with a deflector plate 34, the coolant enters into theannular gap 24 of the upper tube 16 (see FIG. 1A), wherein it ischanneled in a spiral path along the inner wall of this tube 16 to theclosed front end of the latter (see FIG. 1B). Here the coolant passesthrough a communication opening 35 into the annular gap 26 of the lowertube 18, wherein it is channeled in a spiral path along the inner wallof this tube 18 to an outlet chamber 37 with a deflector plate 38 (seeFIG. 1A), which deflects the coolant into an outlet opening 39 in theflange 12. It remains to be noted that in most cases the coolant will bewater, but in specific cases it could be of interest to use a differentcooling fluid than water.

FIGS. 3 and 4 show a teeth support sleeve 40 supporting four rabbleteeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄. This teeth support sleeve 40constitutes with its four rabble teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ aprefabricated unit that is axially slipped on the elongated metallicsupport core 10. In FIG. 1A and FIG. 5, the teeth support sleeve 40 isshown in engagement with the elongated metallic support core 10. It willbe noted that such a teeth support sleeve 40 may have substantially thesame length as the elongated metallic support core 10, so that only oneteeth support sleeve 40 is to be slipped over the elongated metallicsupport core 10. However, for ease of handling, the teeth support sleeve40 will usually be substantially shorter than the elongated metallicsupport core 10, so that several teeth support sleeves 40 have to beslipped one after the other on the elongated metallic support core 10.It is to be understood that a teeth support sleeve 40 may of coursesupport more than four rabble teeth 14 or less than four rabble teeth14, and that it is also possible to conceive a “teeth” support sleevewith a single rabble tooth 14.

A preferred embodiment of the teeth support sleeve 40 includes an innermetallic sleeve 42 and an outer metallic sleeve 44, which are bothpreferably made of stainless steel. As shown on FIG. 5, the innermetallic sleeve 42 has an oval cross-section that provides a form-fitwith the elongated metallic support core 10. An insulating material 46,preferably a micro-porous insulating material, is arranged between theinner steel tube 42 and the outer metallic sleeve 44 to provide a goodthermal insulation.

A preferred embodiment of a rabble tooth 14 will now be described withreference to FIGS. 6 and 7. This rabble tooth 14 consists of a flatelongated ceramic plate, whose first end forms a rabble portion 46, andwhose second end forms a fixing portion 48. The fixing portion includesan ovally shaped through hole through 50 bounded by a rounded off orchamfered edge 52. This through hole 50 is more particularly shaped insuch a way that its edge 52 makes up a form-fit with the outer surfaceof the teeth support sleeve, when the rabble tooth 14 is in it isoperational position on the teeth support sleeve 40. It will be notedthat this outer surface of the teeth support sleeve 40 is advantageouslyformed by a thinner shock absorbing cushioning layer 54 that envelopsthe outer metallic sleeve 44. In summary, the elongated metallic supportcore 10 passes axially through the through hole 50 in the fixing portion48, and the teeth support sleeve 40 co-operates with the fixing portionaround the through hole 50 for fixing the rabble tooth 14 to theelongated metallic support core 10.

Because the rabble tooth 14 has the shape of a simple plate with athrough hole in it, it can be manufactured at reasonable costs in aceramic material that has a good temperature and corrosion resistanceand an excellent wear resistance. Alternatively, only the rabble portion46 may be made of ceramic material, wherein the fixing portion 48 ismade of a cast steel. Such a composite rabble tooth has the advantagethat a cast steel is generally more ductile than a ceramic material andwill thus, under an excessive load, more likely plastically deformitself than break. It will be noted that a deformed rabble tooth may beineffective, but it does at least not fall on the hearth floor, where itwould present a risk for other teeth. In order to warrant a goodconnection between the rabble portion 46 and the fixing portion 48 in acomposite rabble tooth, the latter may include a core made of caststeel. This core extends over the rabble portion 46 and the fixingportion 48 and is provided with a ceramic jacket 55 in the rabbleportion. It is of course also possible to make the rabble tooth 14 ofany other material that has the required temperature, corrosion and wearresistant properties.

Referring again to FIGS. 3 and 4, it will be noted that the fixingportion 48 of the four rabble teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ isembedded in a layer of castable refractory 60, which is cast around theshock absorbing cushioning layer 54. Wire-like armature elements 62 arewelded to the outer metallic sleeve 44, before the refractory 60 is castaround the shock absorbing cushioning layer 54. They protrude throughthe shock absorbing cushioning layer 54 to firmly anchor the refractory60 to the teeth support sleeve 40. In this way, the rabble teeth 14 ₁,14 ₂, 14 ₃ and 14 ₄ can be firmly blocked in their operating position onthe teeth support sleeve 40, wherein the forces acting on the rabbleportion are transmitted via the fixing portion 48 around the throughhole 50 as compressive forces onto the refractory 60 and via the edge 52of the through hole 50 and the shock absorbing cushioning layer 54 tothe teeth support sleeve 40. Additionally, metallic shouldering elements64 (see e.g. FIG. 4) may be welded to the to the outer metallic sleeve44, before the refractory 60 is cast around the shock absorbingcushioning layer 54. In this case the fixing portion 48 around thethrough hole 50 bears on these shouldering elements 64, so that thelatter contribute to the transmission of forces from the rabble teeth 14to the outer metallic sleeve 44. It will further be noted that metallicshouldering elements 64 also warrant that the rabble teeth aremaintained in their operating position even if the refractory 60 isdamaged or falls off.

Teeth support sleeves 40 as shown in FIGS. 3 and 4 can be manufacturedin a workshop ready for being slipped onto the metallic support core 10.As shown on FIG. 1B, the end of the last teeth support sleeve 40′(schematically indicated with a doted line) slipped onto the metallicsupport core 10 is secured to the latter by means of a pin 70. If theteeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ or the refractory 60 are worn out ordamaged, then the teeth support sleeves 40 can be easily slipped offfrom the metallic support core 10 and replaced by new ones. Worn ordamaged teeth support sleeves 40 can be returned to a workshop for beingrefurbished under optimum conditions. It will be appreciated that theexchange of teeth support sleeves 40 can be easily effected from theoutside of the furnace through a maintenance door in the furnace wall,without having to dismount the metallic support core 10 or to enter thefurnace. On FIG. 5, reference numbers 72′ and 72″ refer to withdrawingrods arranged in a free space subsisting between the metallic supportcore 10 teeth support sleeves 40. These two withdrawing rods 72′ and 72″have one end engaged with the first teeth support sleeve 40 slipped ontothe metallic support core 10 and the other end protruding out of thelast teeth support sleeve 40′ slipped onto the metallic support core 10.They allow to easily slip off the teeth support sleeves 40 from themetallic support core 10.

1. A rabble arm for a furnace, said rabble arm comprising: an elongatedmetallic support core; at least one rabble tooth having the form of aplate with a rabble portion and a fixing portion, wherein said fixingportion is formed by a portion of said plate including a through holethrough which said elongated metallic support core axially passes; andfixing means co-operating with said fixing portion around said throughhole for fixing said rabble tooth to said elongated metallic supportcore, said fixing means including an insulated teeth support sleeveslipped over said elongated metallic support core and engaging saidthrough hole in said fixing portion of said rabble tooth.
 2. The rabblearm according to claim 1, wherein said through hole is ovally shaped. 3.The rabble arm according to claim 1, wherein said rabble tooth is madeof a ceramic material.
 4. The rabble arm according to claim 1, whereinsaid fixing portion consists of a first material, and said rabbleportion consists of a second material, wherein said first material ismore ductile than said second material.
 5. The rabble arm according toclaim 4, wherein said first material is a cast steel and said secondmaterial is a ceramic material.
 6. The rabble arm according to claim 1,wherein said rabble tooth includes a core made of cast steel, said coreextending over said fixing portion and said rabble portion and beingprovided with a ceramic jacket in said rabble portion.
 7. The rabble armaccording to claim 1, wherein said teeth support sleeve supports severalrabble teeth by engaging their through holes.
 8. The rabble armaccording to claim 1, wherein said teeth support sleeve provides aform-fit with said elongated metallic support core.
 9. The rabble armaccording to claim 1, wherein said teeth support sleeve provides aform-fit with said through hole of said at least one rabble tooth. 10.The rabble arm according to claim 1, wherein the outer cross-section ofsaid teeth support sleeve and said through hole are both ovally shaped.11. The rabble arm according to claim 1, wherein said teeth supportsleeve includes: an inner metallic sleeve and an outer metallic sleeve;an insulating material between said inner metallic sleeve and said outermetallic sleeve; and a shock absorbing cushioning layer on said outermetallic sleeve, said cushioning layer being engaged by the edge of saidthrough hole in the fixing portion of said rabble tooth.
 12. The rabblearm according to claim 11, wherein said inner metallic sleeve and saidouter metallic sleeve are both made of stainless steel.
 13. The rabblearm according to claim 11, wherein said teeth support sleeve furtherincludes: armature elements protruding from said outer metallic sleevethrough said shock absorbing cushioning layer; and a layer of castablerefractory on said shock absorbing cushioning layer, wherein said fixingportion of said at least one rabble tooth is embedded in said refractorylayer.
 14. The rabble arm according to claim 13, wherein said teethsupport sleeve forms a prefabricated unit, which includes said at leastone rabble tooth and said layer of castable refractory, and saidprefabricated unit is designed to be slipped on said elongated metallicsupport core.
 15. The rabble arm according to claim 6, wherein saidteeth support sleeve includes metallic shouldering elements, and saidfixing portion bears on said shouldering elements.
 16. The rabble armaccording to claim 1, wherein said metallic support core comprises twosuperimposed outer tubes, which are rigidly fixed together.
 17. Therabble arm according to claim 16, wherein said superimposed outer tubesare made of centrifugally cast steel pipes.
 18. The rabble arm accordingto claim 17, wherein each of said outer tubes includes a coaxial innertube, which is arranged in said outer tube so as to delimit therein anannular gap for a coolant flow.
 19. The rabble arm according to claim18, comprising a wire that is arranged is said annular gap so as todefine a spiral flow path for the cool-ant in said annular cooling gap.20. A rabble arm for a furnace, said rabble arm comprising: an elongatedmetallic support core; at least one rabble tooth consisting of a ceramicplate with a rabble portion and a fixing portion including a throughhole through which said elongated metallic support core axially passes;and fixing means including an insulated teeth support sleeve slippedover said elongated metallic support core and co-operating with saidfixing portion around said through hole for fixing said rabble tooth tosaid elongated metallic support core.
 21. A rabble arm for a furnace,said rabble arm comprising: an elongated metallic support core; aplurality of rabble teeth, each of said rabble teeth having a rabbleportion and a fixing portion; fixing means including a teeth supportsleeve slipped over said elongated metallic support core andco-operating with said fixing portions for fixing said rabble teeth tosaid elongated metallic support core, said teeth support sleeveincluding: an inner metallic sleeve, an outer metallic sleeve; and aninsulating material arranged between said inner metallic sleeve and saidouter metallic sleeve, so as to achieve a continuous insulation of theelongated metallic support core.
 22. The rabble arm as claimed in claim21, wherein said inner metallic sleeve provides a form-fit with saidelongated metallic support core.
 23. The rabble arm as claimed in claim21, wherein said insulating material is a micro-porous insulatingmaterial.
 24. The rabble arm as claimed in claim 21, wherein said teethsupport sleeve forms with said rabble teeth a prefabricated unit that isslipped over said elongated metallic support.